The present invention relates to a method for reinforcing structures, more particularly, to a concrete-structure reinforcing method in which the surface of a concrete structure to be reinforced is tightly clad with a fiber sheet or sheets with a resin.
Concrete has been widely used as the principal component of concrete structures such as piers, highway props, architecture, etc. Since concrete has strong alkalinity, the corrosion of the reinforcing iron bars contained in the concrete can be prevented. But in long time use, the concrete loses its alkalinity, initially from the surface portion, and is finally neutralized by the action of carbon dioxide in the air. When concrete neutralization reaches the reinforcing iron bars, they now become susceptible to corrosion and are increased in volume, resulting in causing cracks of the concrete structure to accelerate its deterioration.
The concrete structures are also subject to cracking, collapse, partial loss and other similar troubles by loading a force over the design limitations in the event of an earthquake, land subsidence, overloading or other circumstances. There are also the cases where the strength of the concrete structure is lowered by some reason or other or where the structural strength is insufficient at the stage of design.
In order to overcome these problems, to recover the strength, or to make up for the lack of strength, there have been proposed the concrete structure reinforcing methods in which the concrete surface is clad with a carbon fiber prepreg comprising mainly high-strength carbon fibers and an impregnating agent. For instance, Japanese Patent Application Laid-Open (KOKAI) No. 7-34677 proposes a structure reinforcing method in which as a primer, a cold-setting resin such as epoxy resin, urea resin, resorcinol resin or phenol resin is applied on the surface of a structure to be reinforced, and a carbon fiber prepreg comprising basically carbon fibers having a basis weight of not less than 200 g/m.sup.2 and a resin content of not more than 15% is clad onto the cold-setting resin, this prepreg is further coated with a cold-setting resin such as epoxy resin, and the cold-setting resin is then cured.
In the above reinforcing method, it is essential for elevating the reinforcing effect that the said fiber sheet has a high tensile strength, and that the said sheet is attached fast to the structure surface.
However, in making reinforcement of a structure by using a resin such as mentioned above, the adherent force of the reinforcing carbon fiber sheet layer to the structure surface tends to become into relatively low due to slow curing or incomplete curing of the resin under the low-temperature and water-wet conditions in the winter season. Consequently, the reinforcing fiber sheet layer provided by the reinforcing work may be easily peeled off by an external force, making it unable to produce the desired reinforcing effect.
As a result of the present inventors' earnest studies to solve the above problems, it has been found that when a structure is reinforced by forming a primer layer on its surface by using a specific primer, if necessary, further forming a putty layer on the primer layer, and applying a specific impregnating resin on the surface of the thus formed primer layer (or the putty layer, in case where the putty layer is formed on the primer layer) before and/or after cladding with the fiber sheets, allowing the impregnating resin to penetrate into the spaces in the fiber sheets and curing the impregnating resin, the handling of the impregnating resin applied is greatly facilitated and it becomes possible to increase the strength of the fiber sheets and to attach them fast to the structure surface with a resin, even under the low-temperature and water-wetted conditions in the winter season. The present invention has been attained on the basis of the above finding.